2. Field of the Invention
The invention relates generally to devices for providing attitude rate signals and more particularly to circuits for electronically deriving rate from an attitude displacement signal.
2. Description of the Prior Art
Aircraft attitude rate has long been used in automatic pilots as a basic control signal. Such rate signals are often provided by rate gyroscopes, inertial rate sensors and integrating angular accelerometers. Such rate sensing devices tend to be expensive and although satisfactorily reliable for use in modern automatic pilots, less expensive and still more reliable rate sensors are desirable.
Alternatively, attitude displacement references such as pitch and roll vertical gyroscopes and electronic rate deriving networks have been utilized to provide the attitude rate signal. Although such arrangements are generally less expensive and more reliable than other rate sensing devices, such derived rate networks have been notoriously susceptible to power supply line voltage disturbances resulting in non-attitude related signal components causing undesirable control surface motion. Derived rate networks are particularly susceptible to electrical power supply transients and other noise components because such circuits take the derivative function of the input signal, hence magnifying and sharpening transient noise signals applied thereto. Such disturbances can result in aircraft surface motion resulting in undesired aircraft attitude changes, particularly with modern high speed aircraft having sensitive control surfaces. Attempts have been made in the past to reduce the effects of derived rate circuit noise susceptibility. For example, static compensation has been attempted by utilizing a line voltage compensator in the signal path. Although this effected an improvement as to static compensation, effects due to ripple and transient noise signals still resulted in undesirable non-attitude related outputs, particularly in modern high speed, high performance aircraft where extensive and complex demands are placed on the aircraft electrical systems, thus aggravating the derived rate noise susceptibility. In order to advantageously utilize derived rate networks in aircraft automatic pilot and flight director systems it is important that the attitude signal applied to the rate taker circuit is free of non-attitude related signal components, i.e., the derived rate signal to the aircraft autopilot servos or flight director indicators must be substantially noiseless.
The most significant noise contributions to the rate signals are associated with the aircraft power supply and the various equipment loads and additional noise contributions are associated with the characteristics of the signal generator pickoffs used in the attitude gyroscopes. Typical examples of undesirable noise components which must be compensated for are as follows:
A. Line voltage amplitude ripple. This is normally a continuous amplitude modulation of the aircraft's 400 Hz power supply derived from an engine driven alternator. The modulation is produced by the alternator and variations in the electrical loads placed on the power supply by various electrical equipment in the aircraft, the operating frequencies of such equipment reflecting in corresponding variations in the power supply. The most troublesome ripple frequencies are in the range of 1-20 Hz and those frequencies which produce a beat frequency in this range with the harmonics of 400 Hz since this is the range of maximum aircraft and control system response.
B. Line frequency modulation. The power supply line frequency may change with a variable recurrence rate. This can produce undesirable aircraft control signals in the control response region of 1-20 Hz.
C. Normal line voltage variations, surges and transients. These long term, step and impulse type variations in line voltage amplitude can be produced by increasing and decreasing demands placed on the aircraft power supply by the extensive and complex electrical equipment aboard modern aircraft.
D. Common power and ground connections. When one or more pieces of equipment share the same power or ground wiring as the attitude gyroscope displacement pickoff or the derived rate circuit power input, variable current demands from one equipment will reflect in voltage changes to the derived rate circuit. If the voltage change at the attitude gyroscope displacement pickoff differs from that of the 115 volt, 400 Hz supplied to the power input of the derived rate circuit, an undesirable noise in the rate circuit output may result.
E. Different attitude gyroscope signal pickoff generators. The same derived rate circuits may have to operate with different manufacturers gyroscopes which may include different manufacturers signal pickoff synchros and different phase shift correction networks associated therewith. The non-attitude signal content of the pickoff output may be different from that present in the 115 volt, 400 Hz power supply resulting in undesirable aircraft control surface motion.
Most of the above described noise components may be further aggravated at large aircraft attitude angles due to the increased amplitude of the attitude signals.
Thus it is the object of the present invention to provide a derived rate circuit for aircraft automatic pilots which substantially eliminates the foregoing disturbances so that they are not reflected in motion of the aircraft control surfaces.